How to Spot NASA’s Solar Sail Demonstration Streaking Through the Night Sky

Earlier this year, NASA loaded a microwave-sized device packed with a four-piece, 860-square-foot, ultra-thin solar sail aboard a rocket and launched it on April 23. That same day, the rocket delivered the object—called the Advanced Composite Solar Sail System (ACS3)—into low Earth orbit. After a first unsuccessful attempt, the ACS3 victoriously deployed its booms and unfurled its sails at the end of last month.

“Now that its reflective sail has deployed fully open in orbit, the Advanced Composite Solar Sail System can be seen in the night sky from many locations across the world!” writes NASA’s Arezu Sarvestani. In fact, the ACS3’s reflective surface can at times even appear as bright as Sirius, the brightest in the night sky.

To help individuals spot the ACS3 for themselves, the space agency has added a new capability in the free NASA mobile app for iOS and Android devices. The app provides local schedules of when the spacecraft will be visible to the naked eye and uses augmented reality to direct iOS users toward the appropriate region of the sky.

Shining, shimmering, tumbling

Our Solar Sail is currently orbiting Earth, slowly tumbling as expected while the team characterizes its boom and sails. You can #SpotTheSail intermittently as it passes overhead using our tool in the #NASA_App. Learn more: https://t.co/7jXKqa19Hl pic.twitter.com/XfVetMBNMW

— NASA Ames (@NASAAmes) September 5, 2024

Canadian satellite tracker Kevin Fetter tells SpaceWeather.com that he spotted the ACS3 while it appeared to move past Vega. “It got bright for a short time as it was passing by the star,” he adds. “Briefly it was just as bright as Vega, the 5th brightest star in the night sky.”

Astute observers might notice something strange, however, which multidisciplinary scientist Marco Langbroek spotted earlier this month. In a September 2 blog post accompanied by video footage, he reported changes in the spacecraft’s brightness.

This “indicates [the spacecraft] is now slowly tumbling or wobbling, on a timespan of minutes,” he told Live Science’s Harry Baker last week.

On Thursday, NASA confirmed this observation in a statement, explaining that the ACS3 turned off attitude control, which helps with its orientation, before deploying its booms and sails. This action, the agency says, is in accordance with the planned deployment sequence. In other words, there’s nothing to be worried about.

“Once the mission team finishes characterizing the booms and sail, they will re-engage the spacecraft’s attitude control system, which will stabilize the spacecraft and stop the tumbling,” according to NASA’s statement.

ACS vega

Watch on

Now, the agency’s engineers will test the spacecraft’s capabilities, including its maneuvering abilities. Solar sails work something like regular sails—but instead of relying on wind, they use the push of sunlight. Photons, or particles of light, knock against the solar sail and bounce off, like they’re reflected from a mirror. Each additional photon that hits the sail adds to its momentum, helping the spacecraft gain speed in the vacuum of space.

Solar sails could be advantageous on numerous fronts, per Futurism’s Victor Tangermann. Spacecrafts with solar sails would use a somewhat simple construction and wouldn’t have to rely on heavy and expensive propellants, such as rocket fuel. Solar sails could allow a spacecraft to reach faster speeds than what’s currently available to spacefaring vessels.  

So, when stargazers look up at the moving point of reflected light that is ACS3, what they’re really seeing is a solar sail technology demonstration. NASA will continue to analyze data from the satellite in the coming weeks to better understand this low-cost innovation, with the goal of scaling it up for future space missions.

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.